After the Second World War, so-called corrugated paper boards have been widely spread as packaging materials from the field of paperboard box to the field of wooden caskets. They are also widely used as various cushioning materials. On the other hand, as the progress of mechanization of transportation handling as well as scale-up of machinery, transportation units are growing larger to thereby cause startling changes in transportation circumstances. For example, for conveying cargoes using a fork lift truck, wooden pallet forks have conventionally been used. But, as a part of rationalization in circulation of goods, slip sheets have recently been used in place of these pallets. A slip sheet has a thickness of about 1 to 5 mm, and is made of a sheet of paperboard, fiberboard or synthetic resin sheet, having the same size as the wooden pallet (e.g. 1100.times.1100 mm), provided with a flap portion with a width of about 60 to 120 mm. This flap, which is simply shaped in a form of a sheet being bent in the direction of handling, may disadvantageously be deformed with ease when gripped by the gripper equipped on the fork lift truck. Also, the sheet material itself constituting a slip sheet is deficient in rigidity, and hence cannot stand sufficiently the load of cargoes and may sometimes be deformed to cause sagging of cargoes, etc. Corrugated paper boards are also used as cushioning materials for cargoes. But also, in this case, when the load is too great, the corrugated portion is completely crushed down to render the corrugated board nothing but a mere flat paper board.
For enhancement of rigidity, there may be employed a corrugated sheet made of a synthetic resin to solve the problem all at once. But, when it is bent to form a flap portion, various troubles may be caused during usage due to lack of flexibility at the bent portion. The bent portion is generally formed by heating the portion to be bent by pressing a heat bar on one side thereof, which is in turn bent by about 15.degree. to 30.degree.. Since fusion occurs only at the liner portion on the heated side, the bent portion after cooling is fixed at the bent angle, showing substantially no flexibility for bending. For this reason, when for example a corrugated sheet of a synthetic resin is used as slip sheet, there may sometimes be caused fracture at the bent portion when the flap portion of a slip sheet having loaded cargoes is drawn by gripping with a gripper to be unnaturally deformed. Such a slip sheet is found, for example, in the Japanese Patent Laid-Open Public Disclosure No. 144564/1977. Similar inconveniences may also occur when there are piled on the floor slip sheets having loaded cargoes and further slip sheets having loaded cargoes are to be placed side by side by means of a fork lift truck. In this case, the flap portions protruded from the cargoes on the side of already placed slip sheets (it is necessary for next conveying to have the flap portions thus exposed) may be contacted with those to be newly arranged, whereby the flaps may be pressed by the cargoes to be broken at the bent portions or increased in the bent angle to be made difficult in handling for the next time. The cargoes may also be broken in such a case. These troubles can be avoided by taking sufficient intervals between arrays of cargoes, but storage efficiency is thereby markedly lowered.